Testing done and it makes for interesting reading for anyonbe interested in diplomacy as some things have changed.
To clarify what you are reading
Warmonger points are calculated at the end of each turn and so I will perform an action and the warmonger points get reflected the turn after.
However weirdly warmoinger points seem to degrade 1/turn for the first couple of turns after given and then go to 1/2 turns.
I attack Russia With a Colonial Casus while I am in the modern Era so warmonger points are halved (declaration to a standard civ for this is -12)
I include Gilgamesh as they are an ally and Spain because they hate Russia and Cyrus because he is Nutral... we can then see a difference in warmonger points given based on disliking Russia rather than being my ally although there is a 1 point anomaly at declaration. This is down to Russia and Spain having seen earlier warmongering and so have degrading points in a different turn cycle
Based on this single test I can theorise the following
1. Taking a city now gives 1/4 of initial declaration (unless the code is doubling twice due to colonial)
2. Ceding a city to me still gives me more warmonger points than not ceding (in this case 2 / city)
3. Giving a city back also seems to only remove 2 WM points with the exception of Astrakhan which weirdly gives 4.
4. Capturing a final city gives 3x initial declaration cost
After I have taken Russia I give Moscow to Philip and then the next turn take the city and liberate it to russia (should not be allowed)
This then shows the modifiers
@agonistes mentioned in diplomacy_modifiers.csv as degrading 1 point every 20 turns.
I can share the save if someone wants to recreate... will keep it a couple of days